The colors in images produced on two different image forming devices often differ, even when the image forming devices form images based on the same image data. In other words, if two printers A and B print images based on the same image data, the colors in printed material a produced by the printer A may differ from the colors in printed material b produced by the printer B. These differences in color may be caused by a variety of differences between the two printers, such as the type of printing system (inkjet system, laser system, or the like), type of colorant (pigment ink, dye-based ink, toner, or the like, as well as the manufacturer of the colorant), and type of paper used in the printer, and the type of image processing performed on the image data.
A technique using ICC profiles is well known in the art as a method of color matching designed to produce the same colors in printed materials obtained from different printers. This technique will be described for a case in which the colors of printed material b produced by the printer B are being matched to colors in printed material a produced by the printer A. Here, “printer A” will be referred to as the “target printer” since colors produced by the printer B are being calibrated to match colors in the printed material a, and “printer B” will be referred to as the “output-end printer” since the colors calibrated to match colors in printed material a are outputted by the printer B.
First, a plurality of color images (color patches) for creating a profile is printed on both printers A and B, and a colorimeter is used to measure the colors in each resulting printed material. Next, an ICC profile is created based on the image data and the colorimetric values.
Next, image data used for printing images on the printer B is converted to CIELAB values according to the ICC profile for the printer A. The CIELAB values are then converted to RGB data based on the ICC profile for the printer B so that the printer B can generate printed material b having the same colors as those in the printed material a produced by the printer A.
Relative color matching is performed using the ICC profiles described above, with the color of the paper in regions that colorant is not deposited serving as white, and the color of the black colorant deposited on the paper serving as black. Specifically, a normalization process is performed so that white (the color of the paper) takes on Lab values (100, 0, 0), and black (the black color that can be rendered with the printer) takes on Lab values (0, 0, 0). Hence, white is rendered simply by the color of the paper (i.e., without depositing ink of any color), while black is rendered with the black ink color used by the printer. However, since this normalization process shifts the axis linking black with white, there is an overall shift in all colors other than black and white, resulting in decreased precision in color matching.
It is also possible to perform absolute color matching that does not include the normalization process. This technique achieves high-precision color matching since the shift in colors caused by the normalization process does not occur. However, in absolute color matching, colored ink is visibly deposited on the white paper background when rendering white, as well as when rendering black.
FIG. 7 is a graph showing an example of ranges of colors that can be produced on both printers A and B (color gamut) that are plotted according to the L* and b* axes of the CIELAB color space. The black dots in the upper portion of the graph represent white (RGB=255, 255, 255), and the black dots in the lower portion of the graph represent black (RGB=0, 0, 0).
Thus, when rendering white, the output-end printer B cart produce white that is lighter than the white produced by the target printer A based on the characteristics shown in FIG. 7. Accordingly, the printer B deposits a slight amount of colored ink when producing a white equivalent to that of the printer A. On the other hand, since the printer B cannot render the black color that the printer A can produce, the printer B must use a different color for black. This replacement color is generally a color approaching the target color and in the example of FIG. 7 is depicted by a white dot (unfilled circle). While the printer B renders both black and white properly according to the principles of absolute color matching, this method results in colored ink being visibly deposited on the paper for both white and black.
From the perspective of accurate matching, the rendering of white and black colors in this way will be unpleasant to the viewer. On the other hand, relative color matching causes an overall shift in all colors, resulting in a considerable drop in the precision of color matching compared to absolute color matching.